1. Field of the Invention
The present invention relates to an optical sensor for projecting light rays into a field of detection and detecting information on a subject in the field of detection based on light rays returning from the field of detection.
2. Discussion of the Related Art
There have been various optical sensors such as photoelectric sensors and optical scanners for detecting information on a subject or subjects such as the presence or absence of the subject in a specified field of detection and measurements of the subject, a position of the subject, a configuration of the subject, a distance between the subjects, or a graphical or pictorial image applied to the subject by projecting light rays into the field of detection and detecting an intensity distribution of the light rays that are reflected by or transmit through the subject in the field of detection. Before describing the present invention in detail, reference is made to FIGS. 21 and 22 for the purpose of providing a brief background that will enhance an understanding of the optical sensor.
Referring to FIG. 21 schematically showing a basic construction of a conventional transmission optical sensor, the optical sensor principally comprises a light projection unit 50 and a light detection unit 60. The light projection unit 50 includes a light projection circuit 51, a light emitting element 52 and a projection lens 53. The light projection circuit 51 drives the light emitting element 52 to emit light rays. The light rays emanating from the light emitting element 52 are collimated and then projected on a transparent subject 500 in a specified field of detection 501 by the projection lens 53. The collimated light rays pass through the subject 500 and travel to the light receiving element 60. The light receiving unit 60 includes a detection circuit 61, a light receiving element 62 and a focusing lens 63. The light rays passing through the subject 500 in the field of detection 501 are focused on the light receiving element 62 by the focusing lens 63. The light receiving element 62 converts an intensity of light rays incident thereupon into a voltage signal. The detection circuit 61 detects the presence or absence of the subject 500, dimensions of the subject 500 or a configuration of the subject 500 based on the electric signal from the light receiving element 62.
FIG. 22 schematically shows one of the conventional optical sensors that is disclosed in Japanese Unexamined Patent Publication No. 7-146115, where a light emitting element 101 such as a light emitting diode (LED) emits light rays. A light guide fiber bundle 102 guides the light rays emanating from the light emitting element 101 toward a collimator lens 103. The light rays exiting the light guide fiber bundle 102 are collimated as a parallel beam 112 having a specified beam width by the collimator lens 103 and then they are reflected by a reflection mirror 104. The light rays 112 are projected into a field of detection in which a subject 105 is located and they are partially blocked by the subject 105.
A light-sensitive detector 106 receives the remaining part of the light rays and converts the light rays incident thereupon into an electric signal representative of an intensity of the light rays. A comparator circuit 107 compares a level of the electric signal with a level of a reference signal that is provided by a reference signal generating circuit 108 to determine the presence or absence of the subject 105 in the field of detection based on a result of the comparison and provides a signal representative of a result of the determination.
Generally, a beam of light rays emanating from the LED shows an intensity distribution that is high at a center of the beam and gradually lowers when moving away from the center as shown in FIG. 22. In other words, the light rays projected into the field of detection and received by the light-sensitive detector 106 are uneven and different in intensity depending upon distances from a center axis of the beam. Therefore, if the subject 105 is in a position far from the center axis of the beam where the light rays are low in intensity, the reduction in intensity of the light rays incident upon the light-sensitive detector 106 that is caused by the subject in the field of detection is small. This possibly leads to deterioration of the detection sensitivity and accuracy in some object positions and, in addition, to a failure to detect sizes of some of the subjects 105 in subject positions even though the subjects 105 are the same in size.
In order to provide a wide beam of light rays, it is essential to use a collimator lens having a large aperture, desirably a glass lens, that is comparatively expensive due to the difficulty in precise surface finishing. Although a plastic lens is inexpensive, nevertheless, it is not preferred to use the plastic lens in light of an occurrence of thermal deformation.
A parallel laser beam linear sensor has been known in which a laser diode is used as the light emitting element 101. In this kind of parallel laser beam linear sensor, since the beam of laser rays is strong in intensity, it is allowed to utilize laser rays in a central range of comparatively uniform intensity distribution with a consequence that a parallel beam of laser rays having a comparatively uniform intensity distribution is projected into the field of detection. However, the parallel laser beam linear sensor in which a laser diode is used as the light emitting element 101 is inevitably bulky and expensive.
Referring to FIG. 23 which is showing another one of the conventional optical sensors that is disclosed in Japanese Unexamined Patent Publication No. 7-146115, light rays emanating from a light emitting element 201 are guided to a collimator lens 203 through a light guide fiber bundle 202. The outgoing light rays 211 from the light guide fiber bundle 202 are collimated by the collimator lens 203 and directed as parallel beams of light rays 212 toward a reflective member 213 having a number of total reflection facets 214. The light rays incident upon the reflective member 213 are reflected and divided into a number of parallel beams of light rays 215 by the total reflection facets 214 and then projected into a field of detection. The total reflection facets 214 are arranged so that the beam width of the incoming parallel beams of light rays W1 is expanded to a total width of the outgoing parallel beams of light rays W2.
Since the optical sensor projects expanded parallel beams of light rays into the field of detection, it is allowed to utilize light rays emanating from the light emitting element 201 in a central range with the consequence that a parallel beam of light rays that are distributed almost uniformly in intensity is projected into the field of detection. Therefore, a subject in the field of detection is illuminated with light rays having a uniform intensity irrespective of the subject's position.
However, the outgoing light rays from the reflective member 213 inevitably includes crests and bottoms of intensity distribution occurring correspondingly to pitches at which the reflection facets 214 are arranged. In consequence, since the light rays projected into the field of detection are uneven and different in intensity depending upon distances from a center axis of the beams, if a subject is in a position where light rays are low in intensity, a reduction in intensity of the light rays caused by the subject in the field of detection is small. This possibly leads to deterioration of detection sensitivity and accuracy in some object positions. In addition, light rays emanating from the light emitting element 201 corresponding in position to the bottoms of the intensity distribution are directed not to a subject but reflected in other directions. This inflicts a loss upon light intensity with the consequence that a projection light intensity is lowered across the field of detection, resulting in deterioration of detection sensitivity.